139qmb slow to start

[dan93]

Hi,
New to this forum and new to small engined scooters!

Been building a 20 year old Chinese scoot 139qmb 50cc and have got it running and put on a few hundred miles. Great.

One query I have, when starting it seems to take 6/7 cranks before the engine comes to life. Other times it starts almost instantly, any advice on how to make the start more consistent?

Valve clearance is ideal, and new/used spark plugs don’t make a difference.
I was think perhaps it’s the fuel? Here in Scotland they’ve changed the fuel recently and I’ve heard it’s affected many bikes.

Anything on the subject would be ideal!

Thanks

[GrumpyUnk]

You could have something floating around in the carb that blocks fuel flow to the 'byvalve'. The black cylinder that pokes up vertically from the carburetor is a heat operated needle valve. When cold, it retracts and allows fuel:air mixture to be passed to the intake bypassing the throttle plate. As the engine runs it sends electrical power to the windings, which heat up, and the valve extends and blocks flow. Should it be sticky, you could have difficult cold starts and lack of fast idle. I think the fast idle lasts about 2 minutes and then tapers off.
There are a couple Philips head screws retaining the byvalve to the carburettor(sp) body. It can be removed for inspection. Also there it a tiny tiny aperature from the float bowl to the cylinder the brass tube extends into that feeds the byvalve. If blocked, no fuel will be available for the byvalve to deliver.
tom